Whipstocks have been used for many years in connection with the drilling of boreholes that sidetrack or extend outward from an existing borehole. Although the existing borehole might not be lined with casing (open-hole), typically the hole has been cased so that an elongated window must be milled through the wall of the steel casing to enable a drill bit and string to pass to the outside, In order to form the window, a device known generally as a whipstock is anchored against downward and rotational movement in the casing. A whipstock is primarily an elongated metal cam or wedge having an inclined, concave deflection surface that guides a rotary milling cutter on a drill string while forcing progressive outward movement thereof. The downward and outward movement of the milling cutter, as it is rotated by the drill string, forms an elongated window through the wall of the casing. If desired, a pilot mill can be used first to start the milling cut and/or mill the lug, and then a window mill used to complete the opening. In some cases a so-called "watermelon" mill can be run in tandem or separately from the window mill to ream and finish the edges of the window and ensure that drilling tools run later on will not catch or otherwise hang up in the window. In any event the window permits a drill bit and string, or a drill bit, mud motor and running string, to be advanced therethrough so that a new borehole can be drilled outside the casing.
In many well installations a production string of tubing extends from the surface down inside the larger diameter casing in which a window needs to be formed. A packer usually is positioned near the lower end of the production string to isolate the well bore below the packer from the annulus above it. To remove the packer and the production string from the well, and then reinstall these later, are time consuming and expensive operations which operators seek to avoid where possible. However prior whipstock procedures have necessitated removal of the production string and packer without regard to expense.
An object of this invention is to provide new and improved methods for forming a window in the casing below a production string.
Another object of the present invention is to provide new and improved methods and systems for orienting and setting a whipstock in a laterally offset manner in a casing cement plug below a production string to enable a window to be formed in the casing opposite the deflection face of the whipstock.